Generally, piezoelectric transformers are used in power supplies for televisions, photocopiers, LCD backlights and the like. Prior art piezoelectric transformers are based on the well known Rosen design (U.S. Pat. No. 2,830,274). These prior art high voltage transformer designs are of a piezoelectric ceramic plate which includes a driving section and a driven section which each have different polarizations. The different polarizations provide for voltage transformation.
Piezoelectric transformers are inherently high Q resonators which must be driven at a particular resonant frequency to allow maximum energy transfer to occur for a given output load. There are drawbacks to this, in that, the resonant frequency point of the piezoelectric transformer is dependent on external variables including time, temperature, output load and other variables. These variables cause the optimum power output of the transformer to degrade as frequency shifts unless the driving frequency can be continually corrected. If a driving circuit is unable to track the resonant frequency, the piezoelectric transformer will not be operated most effectively.
Prior art driving circuits have used fixed frequency oscillators with methods to lower the Q of a piezoelectric transformer. Lowering Q, by its very nature, results in less than optimum operating efficiency. In addition, operating at a fixed frequency is inefficient for the reasons outlined above. What is needed is a feedback mechanism to control the driving frequency. However, using voltage feedback is unstable since peak power occurs at a crest of the voltage output curve. In this instance, it is just as easy for a minor voltage feedback variation to adjust the driving frequency upwards as it is to adjust it downwards. This is not desirable, in that, if the feedback causes the frequency to go in the wrong direction, the loop will be 180.degree. off phase and will lock at a low power point.
Phase locked loops are known in the art and have been used to control oscillator frequency. However, phase locked loops have not been applied to controlling piezoelectric transformers. Due to their high-Q nature, piezoelectric transformers require much tighter phase regulation and sensitivity than is available in existing phase locked loops. Typically, phase lock loops compare the phase of two AC signals and provide a DC correction voltage to an AC driving circuit for an oscillator. In addition, phase locked loop circuits typically require loop filters to prevent noise on the DC signal from interfering with the operation of the driving circuit. Also, commercial phase locked loops are complicated circuits particularly adapted for frequency channel switching with quick locking. These capabilities are not required or desired for driving a narrowly confined frequency of a piezoelectric transformer.
A need exists for an improved driving circuit for a piezoelectric transformer which is low cost, uses off-the-shelf components, and provides tight phase regulation and good sensitivity. Also, a need exists for an improved driving circuit for a piezoelectric transformer which does not require DC conversion. In addition, a need exists for an improved driving circuit for a piezoelectric transformer which does need a reference signal and tracks the resonant frequency of the piezoelectric transformer being driven so as to operate at maximum efficiency and/or power output for dynamic loading conditions.